In general the ability to withstand stresses of various kinds decreases with age in animals. This is particularly evident and amenable to study in Drosophila with respect to hyperthermia or heat shock. In this organism a capacity to survive the lethal effects of a single drastic heat shock can be induced by subjecting the animals to a pretreatment at a somewhat lower temperature. We plan to isolate mutants and identify genes involved in the protective action of the pretreatment and to investigate further the molecular basis for the protection. These studies should lead to a better understanding of the age-related changes which form the basis for the reduced capacity to withstand stress.